1. Field of the Invention
This invention relates to improvements in satellite constellations for providing either hemispheric or continuous global coverage.
2. Background and Prior Art
Artificial satellites of earth have been known for some time and are useful in the fields of communication, navigation, surveillance, weather and meteorology, inter alia.
Because individual satellites are expensive and there is considerable cost in the launch and positioning of such satellites, it has been a problem under consideration for over 25 years as to how to distribute a minimum number of satellites in such a way that every point on earth or on a significant area of the earth is continuously in view of at least one satellite.
It is particularly desirable to provide continuous hemispheric coverage, especially as to the Northern Hemisphere so that at least one satellite will be in view at any time from any location in the hemisphere. While multiple satellite constellations are known and are being used to conduct reconnaissance and surveillance and to furnish various services such as communications, navigation, search and rescue for both civilian and military users, at the present time most of these constellations are based on circular geosynchronous satellites in either equatorial or inclined orbits. Since they are in circular orbits, they favor the Northern and Southern Hemispheres equally. However, most of the world's land mass, population, wealth, industry, military assets, and operating areas are located in the Northern Hemisphere.
It is also desirable for some systems to achieve continuous global coverage with a minimum number of satellites. This problem has been addressed for a number of years. In the 1960's it was thought that a minimum of six satellites were required for a constellation to have continuous coverage. In the 1970's it was thought that whole-earth coverage could not be maintained continuously with less than five satellites, see AIAA Paper 84-1996. The prior workers in the field of calculating the number of satellites usually assumed that the constellations would employ circular orbits. While elliptical orbits were recognized, circular orbits were thought to have advantage over elliptical orbits as regards to whole-earth coverage. The determination that at least five satellites are required to provide whole-earth coverage, while true for circular orbits, has been found by the inventor not to apply to elliptic orbits.
Current satellite constellations which use circular synchronous equatorial orbits have coverage gaps at or near the Poles of the earth, regardless of how many satellites are used. For example, three geostationary satellites spaced 120.degree. apart on the Equator leave a triangular gap at both the North and South Poles. For certain applications and missions, it is highly desirable to close one or the other or both of these polar gaps. Generally the Northern Hemisphere is of greater interest since coverage of the North Pole and the entire Northern Hemisphere is quite important for many military, scientific and commercial applications. A commonly known solution resides in adding an extra ring of satellites in inclined orbits. However, this practice can be quite expensive given the high cost of producing, launching and maintaining these extra satellites in orbit. The use of a constellation arrangement which accomplishes the goal of continuous hemispherical or global coverage with the minimum number of satellites thus has obvious economic and operational advantages.